This invention relates to a heat-sensitive recording material. More particularly, the invention relates to a heat-sensitive material including a hydrophilic and hydrophobic solvent resistant protective layer useful in the manufacture of adhesive-backed heat-sensitive labels. The labels are useful in packaging goods which, in transit, storage, or display, may be exposed to such solvents, e.g., meat, produce, or articles of manufacture commonly exposed to water or oleophilic materials. A bar code or alphanumeric information may be formed on such labels at the point of sale by stamping the label with a thermal printing head.
Known recording materials have a thermally imageable layer comprising a binder, a colorless or pale leuco dye, and an acidic substance that causes the dye to change color upon the application of heat. Labels made from such materials are commonly used in grocery stores, delicatessans, and other points of retail sale of commodities sold by weight. Increasingly, they are also used on many other products. At or prior to a sale the retailer weighs the product, commonly on a machine which integrates a scale, register, and thermal print head, and actuates the machine to deliver a thermally improved label indicating the price, weight, and other information in coded and/or alphanumeric form. The label is then affixed to the product, typically by means of a pressure sensitive adhesive backing layer.
Labels of this type are often exposed to water, fats, or oils which can have an adverse effect on the thermal image, increase background discoloration, and in some cases destroy the machine readability of imprinted bar codes. Also, it has been observed that on occasion such labels cause a discoloration to appear on red meat directly beneath the label.
Several attempts have been made to incorporate in thermally sensitive materials a protective barrier layer which can serve to protect the thermal image from the deleterious effects of solvents. For example, U.S. Pat. No. 4,388,362 to Iwata et al. teaches the application of a water-soluble, resinous protective coat over the heat-sensitive layer. Such layers are necessarily sensitive to hydrophilic solvents. U.S. Pat. No. 4,370,370 suggests adding 20 to 100 weight percent of "water-resisting-property-improvement agents" to the water-soluble resin. The result is a mixed resinous system. Another suggested approach involves employing a carboxylated base resin which may subsequently be ionically cross-linked with solutions of aluminum sulfate, iron sulfate, and the like. Again, such protective layers are necessarily subject to hydrophilic solvents because of the water-soluble character of the materials from which they are made.
It is accordingly desirable to provide an adhesive-backed heat-sensitive recording label whose thermal image is protected from background discoloration that may arise from exposure to oils, fats, water, and plasticizers, and which does not reduce meat bloom.